1. Field of the Invention
The present application concerns a method and apparatus for the assay of aqueous liquids containing components to determine their concentration.
2. Discussion of Background Information
Assays of aqueous liquids are undertaken by spectrophotometry in an infrared range of which the wavelength is from 2 to 25 .mu.m.
The main problem for this type of assay is that the water absorption of the infrared radiation in the band indicated (2 to 25 .mu.m) is substantial.
If the intensity of a radiation beam transmitted through a cell filled with water is (Io), and that through a cell filled with aqueous liquid to be assayed is (I), then the absorbency of the components is Log Io/I.
In order to obtain exploitable measurements, the difference between the signal corresponding to the radiation transmitted by a cell filled with liquid to be assayed and that corresponding to the radiation transmitted by the same cell filled with water must be large in proportion to the noise mixed with the measurements taken.
This requires adjustment of the intensity of the incident radiation for each wavelength, or more precisely, for each band of the spectrum corresponding to a function (connection) to be inspected of the components in question.
The adjustment of each band is necessary because, for a polychromatic beam, the energy corresponding to each wavelength is not equal, therefore, such adjustment allows a quantity of energy, for which the signal/noise ratio for the utilized detector is maximum, to pass through the cell.
The intensity adjustment of each band enables one single detector to be used for all the bands, one after another, while working in a sensor area where the signal/noise ratio is high.
Currently, the quantitative analysis by spectrophotometry in an aqueous medium is undertaken by means of monochromatic beams of which the intensity is adjusted.
The word "monochromatic", here means a frequency band whose width is narrow. This is obtainable by means of a polychromatic source cooperating with several optical filters; each one selects and adjusts (attenuates more or less) the intensity of a frequency band such that the energy transmitted for each band corresponds to the sensor area in which the signal/noise ratio is adequately high, in other words, such that the signals measured are exploitable.
Therefore, spectrophotometric quantitative analysis systems in an aqueous medium are dispersive systems. Several monochromatic beams are used; each is obtained from the same polychromatic source and an appropriate filter.
The disadvantages of such systems are:
The number of bands to be inspected corresponds to a number equal to or less than that of the filters; in order to have a spectrum covering the entire range of bands, a very large number of filters is necessary. PA1 The slowness, because the necessary measurements must be undertaken for each band (each filter). In addition, between two successive measurements, it is necessary to wait until the optical and mechanical stability of the system is reached, in other words, one must wait until the mechanical vibrations engendered by the motion involved in changing filters are damped. PA1 to enable all the bands of the spectrum concerning a given component to be taken into consideration, either for a same connection in order to perfect the result to be obtained by introducing the maximum correlation coefficients regarding the connection concerned, or for two or more different connections regarding the same component, PA1 to enable the absorption spectrum to be explored at will for the assay of other components present in the sample; this may lead to more complete analyses. PA1 1/ Sampling at least one water interferogram for the entirety of the predesignated wavelengths, through a water thickness of 10 to 20 .mu.m, such sampling to be undertaken by respecting the following conditions: PA1 2/ Calculating, from the water interferogram, the spectrum (So(f)) corresponding to the intensity (Io) traversing said water thickness as a function of the frequency (f), (So) being the measured values. PA1 3/ Sampling at least one interferogram of said liquid, for the same thickness and under the same conditions as in step 1. PA1 4/ Calculating the spectrum (S(f)), corresponding to the intensity (I) traversing the liquid thickness according to the frequency (f). PA1 5/ Determining, from (So(f)) and (S(f)), the absorbance (A(f)), given A(f)=Log (So/S)=Log (Io/I). PA1 6/ Determining the concentration of each of the components concerned, from the absorbance (A(f)) and by means of the chosen reference analyses. PA1 a temperature of 38.degree. to 42.degree. C. of which the maximum fluctuation is 0.1.degree. C., PA1 a maximum relative hygrometry of 0.2%. PA1 an interferometer producing a polychromatic beam interfered with by successive phase shifts obtained around a zero phase shift, PA1 a measurement container containing: PA1 a calculation and control unit receiving, among others, the signals from the photosensitive detector with respect to the phase shifts and calculating: PA1 a first step constituted by a first heat sensor located in the vicinity of the pump, to detect the temperature (T.sub.1,) of the liquid to be introduced into the measurement container outputting a significant signal of said temperature to the U.C.C., and a first heating means controlled by said U.C.C. to maintain the temperature of the liquid intended for analysis at a temperature (T.sub.1), PA1 a second step constituted by a second heat sensor located in the measurement container, to measure its ambient temperature (T.sub.2) outputting a thermal signal of said temperature to the U.C.C., and a second heating means controlled by the U.C.C. to maintain the temperature of said ambient environment at a temperature (T.sub.2), where T.sub.2 -T.sub.1 =0.5.degree. C., PA1 a third step constituted by a third heat sensor affixed to the cell, to measure its temperature (T.sub.3) and outputting a significant signal of said temperature to the U.C.C., and a third heating means controlled by the U.C.C. to maintain said cell at a temperature (T.sub.3), where T.sub.3 -T.sub.2 =+0.5.degree. C.,
Indeed, in order for these dispersive systems to have an acceptable rate, it is necessary to limit the number of bands to be inspected; this number is currently from 1 to 4 bands for each sample.
The number of samples to be analyzed for the fastest systems is on the order of 300 samples per hour.
Non-dispersive spectrophotometric analysis systems, that is, interferometric systems, are currently used either for qualitative analyses or approximate quantitative analyses for samples where the absorption frequencies of the components are distant from the absorption frequencies of the liquid matrix. The matrix may be a solvent or, in a general manner, the liquid containing the components.
These systems are not currently utilizable for quantitative analyses, especially in the case where the components to be quantified and the matrix absorb the same or very closely related radiation.
The advantage of these systems is speed, because an interferogram involving the entire frequency range is obtained in a single series of measurements.
This interferogram, with the aid of another interferogram corresponding to the matrix, enables the absorbance spectrum to be determined for all the wavelengths of the frequency range.
The major disadvantage of these systems is that a quantitative analysis is not possible in a liquid medium where the matrix and the components to be detected absorb radiation of the same frequency; this is typical in the case of milk for the assay of fats, proteins and lactose.
Indeed, spectrophotometric analysis by interferometry is reputedly impossible for the assay of milk components.